Optical line of sight perimeter protection systems have been proposed in which a laser beam is transmitted to a receiver and reflected to a detector, such that crossing of the beam by an intruder provides an alarm indication. Due to problems of stability and alignment, normally only one reflector is employed, such that each straight line to be protected requires a transmitter-receiver combination. Conventional systems of this type involve a number of disadvantages in addition to their significant cost. These disadvantages include the fact that they require the provision of a power supply to the area to be protected and that they are relatively easily detected by potential intruders with the attendant possibility of being disabled.
Stabilized reflector apparatus is also known for returning a beam in a given direction notwithstanding changes in the position of the reflector along a given axis. Examples of known such reflector apparatus are the penta prism in which the angle between incident and exiting beams is always 90.degree. independent of small changes in the orientation of the penta prism in the plane of the incident and exiting beams. A corner cube prism is also known which is operative to reflect incoming beams by exactly 180.degree. independently of the orientation of the corner cube. The stabilized reflector apparatus which is presently known suffers from the disadvantage that it is either limited in the angle of reflection or it is limited in the axis with respect to rotation about which it is independent.